Heat the oil in a large nonstick skillet over medium-high heat. Add the tempeh and onion; turn the onion occasionally and cook the tempeh until lightly browned, about 4 to 5 minutes per side. Remove from the heat.

Layer each of 4 slices of bread with with 1 slice of cheese, 1 piece of tempeh, ¼ of the onions, ¼ cup sauerkraut, and 1 tomato slice. Top each with a second bread slice, mustard-side down.

Coat a large nonstick skillet with nonstick cooking spray; heat over medium-high heat. Arrange the sandwiches in the skillet, heat until until the cheese is melted and the bread is toasted, about 4 minutes per side.